This invention relates to a system for cancelling offsets in amplifiers. The invention is particularly useful for conditioning a transducer signal.
In Digital Versatile Disc (DVD) and Compact Disc-Read Only Memory (CD-ROM), a photodetector is used for reading information from a disc. The photodetector output current is typically amplified by an amplifier before it is sent to the next stage. "Amplifier offset" is defined as the difference between the DC level of the amplifier output and a reference voltage when no signal (from the photodetector) is applied to the amplifier. Due to the "unipolar" characteristics of the photodetector output current, conventional auto-zeroing circuit may not be used to optimize the amplified output. In existing systems, the amplifier offset is optimized during the wafer sort stage in integrated circuit fabrication by means of a trimming scheme. The integrated circuits for the DVD/CD-ROM systems employ polysilicon or metal fuses. The DC level of the amplifier outputs in such systems are set by applying a high trimming current to burn one or more fuses.
Thus, in a conventional trimming scheme for cancelling the amplifier offset, the cancellation may only be performed once, since after the optimization by burning one or more fuses, the DC level of the amplifier output can no longer be changed. Since different operating conditions may result in different amplifier output DC levels, the above-described conventional trimming scheme is quite limited, since the DC level of the amplifier output cannot be changed with the operating conditions. Thus, the operating conditions may change as a function of the gain setting of the amplifier, the condition of the power supply voltage and temperature, as well as backward or forward compatibility to other devices downstream or upstream in the signal path from the amplifier.
The requirement of the trimming step during the wafer sort stage will add additional testing costs due to the extra testing time required. Moreover, the application of a high trimming current to burn a fuse may, in some situations, affect product reliability. It is, therefore, desirable to provide an improved system for offset optimization where the above-described drawbacks are alleviated or eliminated.